In modern manufacturing methods, for the production of articles of enhanced strength, filament winding techniques are often employed. Such techniques involve the winding of high strength filaments disposed in a matrix of binder material about a form or mandrel of a shape generally corresponding to the required shape of the finished article, and then the curing of the matrix material. In the case of the manufacture of an elongate article such as an airfoil by such filament winding techniques, as the filaments are wound around the form, the form and filaments may be reciprocally displaced relative to one another along the form's longitudinal or winding axis, thereby forming a plurality of plies of filamentary material, the filaments of each ply being angularly offset from the longitudinal or winding axis of the form as well as from the filaments of adjacent plies. To achieve a continuous reciprocation of the filaments along the form as the filaments are wound thereon, turnarounds or winding rings are employed at the ends of the form or the segment of the form being wound, overlying end turns of the various layers being constructed by winding the filaments over the edges and across an outer face of the winding ring while reversing the direction of the longitudinal displacement between the fibers and the form.
Where the article being wound is of a longitudinally tapered shape as are modern wind turbine blades, a continuous winding of the filaments about the form while maintaining mutual reciprocation between the filaments and the form would, as those skilled in the art will recognize, result in a thicker build-up of material at the smaller end of the form than at the larger end. Frequently, this is undesirable. For example, where the wound article is a wind turbine blade to minimize stresses in the blade due to centrifugal loading, it is required that the smaller blade tip be made thinner than the larger blade root. In the prior art, in winding an article such that a smaller tip portion is thinner than a larger root portion, it is the practice to wind the article from a multiplicity of truncated, continuous, filamentary layers. That is, for example, a first complete layer is deposited by winding the filamentary material on the form over substantially the entire length thereof. Thereafter, a second continuous layer is wound about the mandrel over only a portion thereof, the uncovered first layer defining the smaller tip portion of the article being manufactured. Subsequently, a third continuous layer is wound over a portion of the form such that portions of the first and second plies remain exposed. It will be seen that such a method of winding a tapered article of decreasing thickness from larger to smaller ends results in the article having a cross-section or thickness profile of stepped or cascaded shape. With known winding techniques, the height of each step is equivalent to the number of thicknesses of filamentary material required to form a complete, continuous layer. While such a method may allow the manufacturer to roughly approximate an optimum (desired) thickness profile of the article, a method whereby a desired thickness profile is more closely approximated is desirable.
Another method of winding an article while closely controlling the tapering thickness profile is that of utilizing thinner filamentary material (rovings) in the winding operation. However, the requirements of handling such rovings would severely detract from the efficiency of the winding method of manufacture by increasing the number of layers and thus winding time required.